With increased environmental awareness, it is frequently desirable to detect leakage from any large storage vessel, such as a landfill or leachate storage pond for a landfill. Leachate from a landfill, for example, represents the combination of liquid wastes, rainwater, and other undesirable liquids generated by the decay and separation of materials in the landfill. Landfill leachate may leak through a conventional clay liner of the landfill and contaminate groundwater supplies and eventually pollute drinking water supplies. If a leak in a liner is promptly detected, appropriate remedial action may be taken to repair the leak and greatly reduce, or eliminate, the amount of environmental damage resulting therefrom.
Existing landfills are often located very close to residential neighborhoods and new landfill space near major population centers is at a premium. Government regulations for landfills are strict and environmental groups are concerned for non-compliance by landfill operators.
Landfill designs have improved and newer landfills typically include a leachate collection system and a leachate pond. The leachate may then be collected from the landfill and thereby prevented from entering the soil and contaminating the groundwater. However, both the landfill and leachate pond typically cover relatively large geographic areas. Leaks, such as through a conventional clay liner, may occur at almost any point over the large area. Periodic groundwater sampling from the perimeter of the landfill, for example, may not provide early enough detection of a leak to limit the environmental damage caused thereby. Perimeter sampling will also fail to accurately pinpoint the exact location of the leak. Accordingly, what is needed is a reliable, accurate system for quickly detecting and determining the location of subterranean leaks in a large storage vessel, such as a landfill.
The prior art discloses various systems for detecting the moisture content of soil using vertical boreholes and various sensors. For example, U.S. Pat. No. 3,428,806 to Wack discloses an apparatus for determining soil humidity, or moisture content. The apparatus has a neutron source and a gamma ray source and respective detectors for each source. The sources are placed into the bottom of a vertical borehole while two sets of detectors are symmetrically positioned on the surface of the soil. The intensity of radiation detected by the two sets of detectors enables both the humidity per unit of volume of the sample, and its density per unit of volume, to be measured after appropriate calibration. Thus, the humidity per weight can then be determined for the soil sampled.
Radiological, or nuclear, vertical borehole logging is also disclosed, for example, in U.S. Pat. Nos. 3,889,112; 4,430,567 and 4,645,926. U.S. Pat. No. 4,430,567, for example, discloses a neutron generator for induced gamma ray logging wherein the generator is located in a probe which is lowered down into a vertical borehole for determining the porosity of the surrounding geological formation. The probe, as it is lowered or raised, is electrically connected to electronics at the surface of the vertical borehole. Several signals are thereby produced which may be suitably processed to ultimately arrive at a signal which is a direct indicator of geologic formation porosity. Further representative of nuclear vertical borehole logging is U.S. Pat. No. 4,665,486 to Schultz which discloses a downhole sonde, including a neutron generator and radiation detectors, which is lowered into the borehole while information is collected topside via an electrical logging cable.
U.S. Pat. No. 4,754,136 to Blakely discloses a system for detecting leakage from a buried tank containing a liquid organic material, such as gasoline. A series of vertical boreholes are provided around the perimeter of the tank. Measurements of the content of volatile organic material in the soil surrounding the series of vertical boreholes are taken by lowering a neutron backscatter gauge into the vertical boreholes.
Unfortunately, the vertical borehole logging systems of the prior art are not generally suitable for detecting moisture content in the soil beneath a relatively large storage vessel. An accurate determination of leaks from the interior area of the vessel can not be made by vertical borehole sampling around the perimeter of the storage vessel. A series of vertical boreholes drilled in the interior of the landfill are impractical and would create undesirable penetrations of the clay liner, perhaps causing an additional leakage problem. An impractically large number of vertical boreholes may be required to adequately monitor a large area, such as that covered by a typical landfill.